Electromechanics

AC machines

Let’s consider the electric machine in Figure 1 (with windings as shown). Let’s assume that the winding coils contain three-phase voltage. The picture showing currents through the windings is depicted in Figure 2.  The currents are sinusoidal :

${i}_{1}=I\mathrm{cos}\left({w}_{e}t\right)$ ${i}_{2}=I\mathrm{cos}\left({w}_{e}t–\frac{2\mathrm{\pi }}{3}\right)$ ${i}_{3}=I\mathrm{cos}\left({w}_{e}t–\frac{4\mathrm{\pi }}{3}\right)$

Magnetic flux, generated by the AC machines, will rotate in time and space. The rotation speed is related to the frequency of currents through the machines windings. The speed of the rotating magnetic field depends on the frequency of excitation current and the number of poles on the stator, and is called synchronous speed.

Figure 1. The AC electric machine scheme.

Figure 2.The current curves for the AC electric machine.

The rotor speed depends on the magnetic poles in the stator and the rotor. The torque value depends on the angle between stator and rotor magnetic fields. The number of poles in the AC machines should be multiple to two.